The present disclosure relates to an image forming apparatus equipped with a looped belt and a method of detecting an index.
Some image forming apparatuses are equipped with, for example, a plurality of photoconductor drums so as to be capable of performing color printing using toner images of a plurality of mutually different colors. In such image forming apparatuses, endless belts, such as an intermediate transfer belt and a sheet transportation belt, may be used when each of toner images borne by the surface of the photoconductor drum is transferred onto a sheet of paper.
In order to determine the timing at which, for example, the toner image is formed or a sheet is transported, the position of the belt, in the rotational direction, needs to be precisely detected. In order to detect the position of the belt in the rotational direction, an index may be provided at part of one of the end edges of the belt. In this situation, the index is rotated together with the end edge of the belt. In such a structure, a detecting member is located in a rotational path of the index. The detecting member detects the index, thereby allowing the position of the belt in the rotational direction to be detected.
Assuming that, as an example, the detecting member uses a through-beam sensor. In this situation, when the index passes through the detection area of the detecting member, the amount of light blocked in the detection area is increased, and when the index has completely passed through the detection area of the detecting member, the amount of light blocked in the detection area is decreased. That is, the output value of the detecting member is different depending on whether or not the index is passing through the detection area. Thus, the index can be detected based on the output value of the detecting member.
An endless belt, such as the intermediate transfer belt or the sheet transportation belt, rotates while being looped over a plurality of rollers. However, the belt may meander due to causes such as insufficient precision in the manufacture or installation of the rollers, shifting of the axes of these rollers, a non-uniform thickness of the belt, a non-uniform tensile force by which the intermediate transfer belt is stretched, and so forth. That is, the end edge positions of the belt may move in a direction wherein the end edge positions move close to or away from the detecting member. Accordingly, when the belt meanders, the precision with which the index is detected may be degraded.
Specifically, a threshold value, which serves as a reference with which whether or not the index is present in the detection area of the detecting member is determined, is typically a fixed value. For example, assuming that a fixed threshold value is set, such that the timing at which the output value of the detecting member changes by a specified amount matches the timing at which the index is detected when the belt does not meander, that is, the end edges of the belt are in an ideal state wherein the end edges of the belt are not shifted from the set rotational path of the belt. In this situation, when the belt meanders, the output level of the detecting member generally changes while the threshold value remains at a fixed level. When the detecting member has, for example, a through-beam sensor, the output level of the detecting member is generally decreased or increased because the amount of light that is blocked in the detection area increases when the end edges of the belt move in the direction wherein the end edges move close to the detecting member, and the amount of light that is blocked in the detection area decreases when the end edges of the belt move in the direction wherein the end edges move away from the detecting member.
Accordingly, in the situation wherein the output waveform (amount of change in output value) of the detecting member is changed into a rounded waveform while the index is passing through the detection area, the index is detected when the output value of the detecting member has changed by an amount greater than the above-described specified amount, or when the output value of the detecting member has changed by an amount smaller than the above-described specified amount. That is, precision with which the index is detected may be degraded.